Are your 8" vents round? If so then drilling the hole may not be possible, I don't believe anyone makes drill bits for such a large diameter. A good glass shop should be able to cut a circular hole for you, and if you go that route then I would do the single vent in the top and skip the front vent, an 8" opening should allow for plenty of airflow. Do your vents have screen in them, or are they louvered aluminum?